iodination of alkylbenzenes using ...downloads.hindawi.com/journals/jchem/2009/928040.pdfiodination...

.

ISSN: 0973-4945; CODEN ECJHAO

E-Journal of Chemistry

http://www.e-journals.net 2009, 6(4), 1164-1166

Iodination of Alkylbenzenes using

Benzyltriphenylphosphonium Dichromate / I2 System

R. SATHIYAPRIYA and R. JOEL KARUNAKARAN*

Department of Chemistry, Madras Christian College, Tambaram,

Chennai -600 059, Tamilnadu, India.

[email protected]

Received 26 February 2009; Accepted 4 April 2009

Abstract: An efficient and rapid method have been found using

benzyltriphenylphosphonium dichromate (PhCH2PPh3)2Cr2O7 as an oxidant

along with molecular iodine for the iodination of aromatic compounds.

Iodination of a wide variety of alkyl benzenes were done under

dichloromethane solvent system and high yields of iodo products were obtained

Keywords: Aromatic iodination, Benzyltriphenylphosphonium dichromate, Alkyl benzenes.

Introduction

The aromatic iodination reactions are important electrophilic substitution reactions and the

resultant iodoarenes have been widely used in organic synthesis due to their ability to

undergo metal catalyzed cross-coupling reactions1. Iodoarenes are in many ways similar to

bromo and chloro arenes in chemical properties except of their ability to form stable

polyvalent iodine derivatives, which are versatile reagents in organic synthesis2. Aromatic

iodides are widely used in the production of organic photoreceptors that are used in

electrophotography, organic electroluminescent device, dyestuffs and agricultural chemicals.

Polyiodinated phenols have been used as imaging agents in noninvasive medical diagnostic

techniques3. Since iodine is the least reactive one among the halogens, iodination of

aromatic compounds simply by diiodine is not possible. Hence aromatic iodination reactions

were done by using iodonium ion donating reagents such as iodine monochloride4, NaI-

chloramine-T5 and N-iodosuccinimide

6 or by oxidative iodination method using oxidants

along with molecular iodine. Some of the oxidants reported7 are Pb(OAc)4, sodium nitrate

8,

trimethyl orthoformate9, oxon

10, silver(I)sulfate

11, KMnO4

12 and peracetic acid

13,

KClO3 14

, KBrO3 15

and NaIO316

. As a part of on going synthetic project, an efficient and

rapid method have been found using benzyltriphenylphosphonium dichromate

(PhCH2PPh3)2Cr2O7 as an oxidant along with molecular iodine for the iodination of alkyl

benzenes were done in dichloromethane solvent system.

X

I

X

mol equiv. of benzylphenyl 0.1

phosphnium dichromate

mol equiv. of iodine0.5

CH2Cl2

X=alkyl

Iodination of Alkylbenzenes 1165

Experimental

TLC (Thin layer chromatography) was performed on silica gel-G coated aluminium plate

(Merck 60, F-254) using hexane as eluent. The chromatogram was developed under a

mixture of 1% vaniline and 5% ethanolic H2SO4 as reagent. Melting points were obtained

on a Thomas-Hoover apparatus in open capillary tubes and are uncorrected. 1H NMR and

13C NMR spectra were acquired on JEOL 270 and 400 MHz and Varian Gemini 300 MHz

Spectrophotometers. Mass spectra were recorded on a GC-Mass Spec Finnigan MAT

8230MS instrument.

Preparation of benzyltriphenylphosphonium dichromate

Benzyltriphenylphosphonium dichromate can be prepared easily by mixing an aqueous

solution of benzyltriphenylphosphonium chloride with chromium(VI)oxide in HCl at room

temperature17

. It is a stable orange coloured powder which can be stored for a month and has

the solubility in dichloromethane.

Typical iodination procedure

o-Xylene (1.06 g, 10 mmol) was added to a stirred solution of the benzyltriphenyl-

phosphonium dichromate (1 mmol, 0.92 g) and iodine (5 mmol, 0.63 g) in 40 mL

dichloromethane. The mixture was heated at reflux until TLC showed complete disappearance

of starting material which required 1-2 h depending on the reactivity of the substrate. The

reaction mixture was cooled to room temperature and then poured into vigorously stirred aq.

Na2SO3 solutions (5 g Na2SO3 in 50 mL of water). The organic layers were separated, dried

over anhydrous Na2SO4, filtered, and the removel of solvent was worked up by evaporation.

The residue was purified by column chromatography on silica gel using hexane as eluent to

afford 1-iodo-3,4-dimethylbenzene (2.20 g, 95%), b.p.234 oC (lit. 231 – 233

oC).

MS (m/e) : 232 ([M+] 97%).

1H NMR (CDCl 3): δ 2.24 (s, 3H), 2.38 (s, 3H), 6.73 (d,

J = 8.4 Hz, 1H), 7.02 (dd, J = 8.4 Hz and 2.3 Hz, 1 H), 7.61 (d, J = 2.3 Hz, 1H). 13

C NMR

(CDCl 3): δ 22.4, 23.5, 97.4, 131.3, 134.2, 135.1, 137.2, 138.8.

Results and Discussion

Initially, iodination of o-xylene was carried out with a mixture of benzyltriphenyl-

phosphonium dichromate and iodine in dichloromethane according to the following

stoichiometry under reflux condition (Eq.1).

(1)

The crude iodinated product obtained by this method were purified by column

chromatography over silica gel and authenticated by TLC. Finally the iodo product was

established by mass, 1H NMR and

13C NMR spectral studies. It was observed that o-xylene

gave excellent yields of 1-iodo-3,4-dimethylbenzene.

Further a set of alkyl substituted benzenes were subjected to this iodination process and

monoiodination was achieved in each case with excellent yield of iodo product. The results

are summarized in Table 1. The possible reactions can be explained as follows. The oxidant

above said is able to convert molecular iodine into more reactive electrophilic I+ species.

This in situ formed iodonium ion may lead to the iodination of aromatic compounds.

1166 R. JOEL KARUNAKARAN et al.

Table 1. Iodination of alkyl benzenes with benzyltriphenylphosphonium dichromate and iodine.

a Products were characterised by m.p or b.p, Mass, 1H NMR and 13C NMR. b Isolated yield.

Conclusion

An inexpensive oxidant benzyltriphenylphosphonium dichromate can easily be prepared in

laboratory at room temperature and can be used in the oxidative iodination process. This

methodology is rapid, inexpensive and superior to previously reported methods in terms of

yield and purity of products.

References

1. Diederich F and Stang P J, Metal Catalyzed Cross-coupling Reactions; Wiley-VCH:

Weinheim, Germany, 1998.

2. Stang P I and Zhdankin V V, Chem Rev., 1996, 96, 1123.

3. Papa D, Schwenk E, Breiger H and Peterson V, J Am Chem Soc., 1950, 72, 2619.

4. Chaikovskii V K and Filimonov V D, Russ J Org Chem., 2001, 37, 1130.

5. Hatanaka Y, Hashimoto M, Kurihara H, Nakayama H and Kanaoka Y A, J Org

Chem., 1994, 59, 383.

6. Carreno M C, Ruano J L T, Sanz G, Toledo M A and Urbano A, Tetrahedron Lett.,

1996, 37, 4081.

7. Serguchev Yu, Davydowa V G, Makhon’kov D I, Cherprakov A V and Beletskaya I P,

J Org Chem., USSR, 1985, 21, 1841.

8. Yusubov M S, Filimonov V D, Jin H W and Chi K W, Bull Korean Chem Soc., 1998,

19, 400.

9. Tatsuya Shono, Yoshihiro Matsumura, Sasumu Katoh, Kaoru Ikeda and Tohru

Kamada, Tetrahedron Lett. 1989, 30, 1649.

10. Narender N, Srinivasu P, Kulkarni S J and Raghavan K V, Synth Commun., 2002, 32, 2319.

11. Brown E and Robin J P, Tetrahedron Lett., 1977, 2015-2018.

12. Chaikovskii V K and Novikov V T, Zh Prikl Khim., 1984, 57, 134.

13. Ogata Y and Nakajima K, Tetrahedron, 1964, 20, 43.

14. Sathiyapriya R and Joel Karunakaran R, Synth commun., 2006, 36, 1915.

15. Sathiyapriya R and Joel Karunakaran R, J Chem Res., 2006, 575.

16. Sathiyapriya R and Joel Karunakaran R, E J Chem., 2006, 3(2), 65-67.

17. Abdol Reza Hajipour and Shadpour E Mallakpour, J Chem Res., 2000, 32.

Substrate aProduct M.p (lit),

oC

bYield, %

Toluene 4-Iodotoluene 33(32-35) 92

m-Xylene 1-Iodo-2,4-dimethylbenzene b.p.110(112) 94

o-Xylene 1-Iodo-3,4-dimethylbenzene b.p.234(233) 95

p-Xylene 2-Iodo-1,4-dimethylbenzene b.p.112(113) 85

Mesitylene 2-Iodo-1,3,5-trimethylbenzene 30(31) 93

Ethylbenzene 4-Ethyliodobenzene b.p.112(113) 86

Submit your manuscripts athttp://www.hindawi.com

Hindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

Inorganic ChemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

International Journal ofPhotoenergy


Carbohydrate Chemistry

International Journal of


Journal of

Chemistry


Advances in

Physical Chemistry

Hindawi Publishing Corporationhttp://www.hindawi.com

Analytical Methods in Chemistry

Journal of

Volume 2014

Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttp://www.hindawi.com Volume 2014

SpectroscopyInternational Journal of


The Scientific World JournalHindawi Publishing Corporation http://www.hindawi.com Volume 2014

Medicinal ChemistryInternational Journal of


Chromatography Research International


Applied ChemistryJournal of



Theoretical ChemistryJournal of


Journal of

Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International


CatalystsJournal of

ElectrochemistryInternational Journal of

Hindawi Publishing Corporation http://www.hindawi.com Volume 2014

iodination of alkylbenzenes using ...downloads.hindawi.com/journals/jchem/2009/928040.pdfiodination...

Documents